Typical proximity exposure apparatuses, which perform exposure by bringing a mask (master) and a substrate such as a wafer, or the like, close to each other, include an X-ray exposure apparatus. For example, an X-ray exposure apparatus using an SR light source is disclosed in Japanese Patent Laid-Open No. 2-100311.
FIG. 1 is a schematic view showing a general arrangement of a conventional X-ray exposure apparatus of this type. In FIG. 1, a mask 101 with a patterned mask membrane 102 is held by a mask chuck 104 mounted on a mask stage base 106 and aligned with respect to an X-ray optical path. A wafer 103 is held by a wafer chuck 105, faces the mask 101, and is spaced apart from the mask 101 by an infinitesimal distance, i.e., arranged close to the mask 101. The wafer chuck 105 is mounted on a fine adjustment stage 113 used to align the mask 101 and wafer 103. The wafer chuck 105 and fine adjustment stage 113 are mounted on a coarse adjustment stage 112 used for movement between shots so that the irradiation region of X-ray beams can be sequentially stepped over a plurality of field angles of exposure of the wafer 103. The coarse adjustment stage 112 is guided by a stage base 107. An alignment scope 108 is designed to measure the amount of shift between the mask 101 and the wafer 103 in their alignment and is mounted on an alignment stage 109. The alignment stage 109 is mounted on the mask stage base 106 and is used to move alignment light emitted from the alignment scope 108 to an alignment mark position (not shown) formed on the mask membrane 102.
Generally, in an X-ray exposure apparatus, the mask membrane 102 and wafer 103 are spaced apart from each other by an infinitesimal distance of 10 to 30 μm to face each other, and exposure (proximity exposure) is performed using the step & repeat scheme, in which exposure of the wafer 103 to the pattern on the mask membrane 102 is repeated a plurality of number of times.
The procedure for performing exposure by global alignment in this conventional X-ray exposure apparatus will be described below.
(1) The coarse adjustment stage 112 is driven such that the first shot of the wafer 103 in global alignment is located below the mask membrane 102.
(2) The fine adjustment stage 113 drives the wafer 103 such that the distance (to be referred to as a gap hereinafter) between the mask 101 and the wafer 103 changes from the gap for stepping to the gap for gap measurement and performs gap measurement by the alignment scope 108.
(3) After the fine adjustment stage 113 makes the wafer 103 parallel to the mask 101, a measuring unit (not shown) measures a shift in the in-plane direction between the mask 101 and the wafer 103 at a plurality of points, and a controller (not shown) calculates the correction amount of the positional shift of each shot.
(4) The coarse adjustment stage 112 drives the wafer 103 such that the first shot of the wafer 103 in exposure is located below the mask membrane 102. After the fine adjustment stage 113 corrects the in-plane positional shift of the shot, the fine adjustment stage 113 adjusts the gap so as to equal the gap for exposure.
(5) The X-ray exposure apparatus performs exposure.
(6) The fine adjustment stage 113 adjusts the gap so as to equal the gap for stepping, and the coarse adjustment stage 112 steps the wafer 103 to the second shot in exposure.
The X-ray exposure apparatus performs exposure for a predetermined number of shots of the wafer 103 by repeating the steps (4) to (6) in the same manner.
However, a conventional X-ray exposure apparatus does not take any measurement error induced by a wafer process into consideration in gap measurement, posing the following problems.
When gap setting is performed on the basis of the measurement result including any measurement error induced by the wafer process, an error occurs in gap setting by the magnitude corresponding to the measurement error. As a result, imaging performance degrades and overlay accuracy decreases. Note that in this specification, measurement errors induced by the process include ones due to unevenness of the wafer surface (e.g., unevenness of the pattern, defects due to a foreign substance, roughness of the wafer surface, unevenness of the reverse surface of the photoresist applied to the wafer surface, and the like). Additionally, these problems are not limited to the proximity scheme. For example, similar problems arise in, e.g., AF measurement by reduction projection exposure using an excimer laser as a light source.
Generally, in reduction projection exposure, AF measurement is performed by diagonally projecting light onto the wafer surface and detecting its reflection light as the height of the wafer surface using a CCD, or the like. In this method as well, the wafer process induces measurement errors. For this reason, a preceding wafer is used to perform pre-exposure, thereby determining the best focus from the exposure result, for each wafer layer (exposure step). In actual exposure, any measurement error is reflected as an offset value in AF measurement or AF setting on the basis of the best focus.
However, as described above, a method of exposing a preceding wafer to obtain an offset value poses a problem that the operating time of the exposure apparatus shortens to reduce the productivity of devices.